In case it helps anyone else I found that terminating appstoreagent and deleting ~Library/Caches/com.apple.appstoreagent enabled me to download from TestFlight again. Installing my production app from the App Store still fails though - it downloads but then fails at the last step.

Yes this sounds exactly the same. I didn't mention it in my original post but we also have 2 iOS targets as part of the project and those always install fine. This is 100% a macOS issue.

I'm having this issue too. Did you find a solution?

Hi. Is there any update on this? It seemed to be fixed for a few weeks (or at least we didn’t get any problem reports) but over the last few days we’ve had lots of users reporting the issue again.

It's already there in my original post: FB11733455

Hi @JoeKun. I’ve just opened FB10030197 for this. It contains a detailed write up, including log files, which demonstrates the issue. It’s a very real problem: you add tracks to a playlist, get a 204 back (success), then query the playlist and the tracks are not there. I actually opened a TSI for this 2 days ago as it’s critical to my business. So far I’ve heard nothing back.

I discovered this purely by chance (and I shudder to think about what is going under the covers) but I found that if the following line of code gets run before the call to ApplicationMusicPlayer then it all seems to work: let _ = MPMediaQuery.playlists() Clearly this ought not to have any effect at all, but it works for me so I'm sharing it in case it works for others.

Thank you. I wasted an entire day trying to find a non-existent bug in my code before finding this comment. Adding this one modifier to the Navigation view's parent fixed everything.